Kaganowicz, in a copending application entitled, "METHOD FOR PREPARING AN ABRASIVE COATING", Ser. No. 963,819, filed Nov. 27, 1978, and incorporated herein by reference, discloses a method for preparing an abrasive silicon oxide coating, SiO.sub.x, where 1.ltoreq.x.ltoreq.2, on a substrate comprising glow discharging silane and a gaseous, oxygen-containing compound chosen from the group consisting of N.sub.2 O, H.sub.2 O and CO.sub.2. This coating is generally comparatively thin, e.g., on the order of 250 angstroms thick.
A thicker SiO.sub.x coating is sometimes needed. For example, Kaganowicz et al., in another copending application entitled, "A METHOD OF PREPARING AN ABRASIVE-COATED SUBSTRATE", Ser. No. 255,503 which is being filed concurrently with the present application and is incorporated herein by reference, teaches that the thickness and the refractive index of the SiO.sub.x coating influence its abrasive properties. Generally, a refractive index of between about 1.1 and 1.45 angstroms and a coating thickness of at least about 600 angstroms are preferred.
Dholakia, in U.S. Pat. No. 4,243,395 teaches a method of lapping the tip of a hard material having a sharp point which includes using an abrasive SiO.sub.x layer thick enough to prevent penetration by the sharp point through the layer to the substrate. For example, for a diamond having a point with a radius of from about 0.1 to about 0.01 micrometer or less a layer about 2000 angstroms thick may be required.
When a thick coating is required, the glow discharge deposition process may sometimes have to be interrupted, for example, to allow the substrate to cool, Wang et al., in U.S. Pat. No. 4,260,647, entitled, "METHOD FOR DEPOSITING AN ABRASIVE LAYER", issued Apr. 17, 1981, teach a method for preparing a thick abrasive silicon oxide layer from an organosilane and oxygen. According to Wang et al., when the deposition is interrupted, the previously deposited SiO.sub.x layer should be subject to a glow discharge in the presence of oxygen prior to resumption of the glow discharge deposition of SiO.sub.x.
Because of the vigorous, spontaneous interaction between O.sub.2 and SiH.sub.4, the Wang et al. method is not attractive to use for SiH.sub.4 as a starting material. It would therefore be desirable to have a method of glow discharge depositing a thick layer of SiO.sub.x from SiH.sub.4 that does not require the glow discharge of oxygen and which leads to more abrasive coatings.